Electrical protective device



June 9,1931.

H. E. BUTLER 1,809,673

ELECTRICAL PROTECTIVE DEVICE Filed May 21, 1930 figl.

--L mmma His Attorney Patented June 9, 1931 UNITED STATES PATENT OFFICEHENRY E. BUTLER, OF SCOTIA,'NEW YORK, ASSIGNOR '10 GENERAL ELECTRIC GOM-IPANY, A CORPORATION OF NEW YORK Application filed May 21, 1930. SerialNo. 454,499.

My invention relates to protective devices used in electrical circuitsto prevent excessive flow of current therethrough.

In electrical circuits where a plurality of,

burns out, the condition is met with that when a lamp burns out and itscut-out operates, the resistance of the circuit is lowered by an amountequal to that of the lamp filament burned out, which results in anincrease in the amount of current flowing through the circuit, theeffect of which is to cause the remaining lamps to burn' outmuch sooner.This is particularly true in the small series lamp circuits used forChristmas tree lightmg.

One object of my invention is to provide for use in connection with astring of lamps connected in series, the lamps being provided withcut-outs, an automatic current limiting device which, when a lamp burnsout, operates to insert in the series circuit a resistance of a valuesuch as to limit by the desired amount the current which will flow inthe circuit. The resistance may be of a value substantially equal to theresistance of the lamp filament in which case the remaining lamps willburn with their normal brightness, 01 the resistance may be somewhatgreater than the resistance of the filament, for example, 10% greater,in which case the remaining lamps in a series circuit will be 5 dimmedsomewhat, thus indicating that there is a lamp in the string which hasburned out.

A further object of my invention is to provide an automatic currentlimiting device which is simple in structure, effective in operation,easy to manufacture, and which may be used in place of the plug capdisposed at the terminal end of a string of series connected lamps. Fora consideration of what I believe to be novel and my invention attentionis directed o: the following description and the claims pended thereto.p the accompanyingdrawings showing il'];us'trative embodiments of myinvention, {and in which the same numerals refer to the ends arethreaded inwardly through persimilar parts in the several views, Fig. 1is a diagrammatic illustration of a series lamp circuit with a deviceembodying my invention connected therein; Fig. 2 is a longitudinal viewin section of a device shovving'one embodiment of my invention, and Fig.3 is a longitudinal view in section showing a modified embodimentthereof. I

In Fig. 1 of the drawings, 28 indicates a number of incandescentelectric lamps connected in series, each lamp being provided with acut-out 29 which, in case the filament of a lamp burns out, operates dueto the increased voltage to which the lamp is then subjectedtocloseashuntcircuit around the lamp. Any suitable lamp structureembodying a cut-out may be used. The cut-out indicated in the presentinstance may comprise a wire coated with oxide, for example, a copperwire coated with copper oxide, and connected '10 across the leading-inwires of the lamp, Normally, the oxide acts as an insulator but, in casethe lamp is subjected to increased voltage such as occurs in case afilament burns out, the oxide breaks down thus forming a shunt circuitacross the leading-in wires of the lamp. It will be understood that thetype of lamp and cut-out shown is only illustrative but my invention maybe carried out in connection with any type of lamp provided with acut-out.

Referring to Fig. 2, 4.- represents a cylindrical casing molded orformed out of a suitable insulating material, and having the left endclosed by an integral end piece 5. The right end is closed by a disc'6which fits snugly within the end of the casing.

A helical groove 3 is cut or formed around the outside of casing 4intermediate its ends. A length of resistance wire 7 having a knownresistance, for example, a resistance equal to or of an order about tenpercent greater than the resistance of one of the lamps forming thecircuit is wound in the helical groove 6 and forations which pierce thecasing at the ends of the helical gro0ve.

Contacts 8 adapted to be inserted into a standard plug or service outletextend outwardly through spaced apertures in the end 5.

The upper contact is provided by an inner lug 9 which acts as a shoulderto abut against the inner face of end 5. The lower contact is thecontinuation of a horizontal metal strip 10 which has an upwardlyprojecting offset 11 forming a shoulder, which abuts against the innerface of end 5. The contacts are held rigidly in place by having rings 12forced over the outer end of each contact and inwardly until they abutagainst the outer face of end 5, where by frictionally gripping theouter surface of the contacts 8 they prevent any axial movementsthereof. The strip 10 is bent transversely upwards at its right end toform a flange 13 against which the disc 6 abuts and where it is held inplace by a terminal screw 17 passing through a perforation' therein, thehead abutting against the outer face of the disc and the end of theshank being threaded into a tapped hole through the flange 13.

In order to have a series cutout or contact member sensitive to smallincreases in current, such as would be caused by the burning out of oneof the lamps, a thin metal thermostat 14 is soldered or braced to thelug 9. The thermostat extends from the lug downward to about the axis ofthe casing 4 where it is bent transversely and extends axially towardthe right for the greater length of the inner casing, where it makescontact with spring 14. The thermostat is of such cross section thatwhen the current flowing through it exceeds a predetermined amount, itis adapted to bend upward, breaking the electri cal contact between itand spring 15. Spring 15 is attached to the upper inner face of disc 6by a nut 17 which is threaded on a terminal screw 16 which passesthrough the aligned perforations in the disc 6 and in the upper end ofspring 15, the head of the screw abut ting against the outer face ofdisc 6. Spring 15 extends downward along the inner face of disc 6 to apoint in axial alignment with thermostat 14 where it is benttransversely inward.

The end of thermostat 14 is curved slightly upward, while the end ofspring 15 is curved slightly downward, thereby facilitating theassembling of the protective device.

The series cut-out must be sov arranged that the electrical circuit willnot be broken when the thermostatic cut-out opens. To effect this end,and at the same time limit the current within the lamp circuit and thusprotect the lamps still burning, the left end of resistance wire 7 issoldered to the lug 9 while the right end is soldered to the nut 17,thus connecting the resistance wire in parallel with the thermostat insuch a mannerthat when the thermostat bends upward due to the heatingcaused by the flow of'excessivecurrent therethrough opening theelectrical current through it, the resistance 7 will be in series withthe lamps.

To provide a dependable means for keeping the series cut-out open when alamp has burned out, or after the series cut-out has operated, a smallarmature 18 having an end flange 19, which is soldered to the top ofthermostat 14, extends upwardly and out through a radial hold in thecasing 4. A small ring magnet 20 surrounds the armature 18 and issecurely held within an inner enlarged portion of the aperture throughwhich the armature projects. A second flange 21 is attached to thearmature 18 at a point just below the lower face of ring magnet 20, andat a distance where magnetic attraction is a little less than thatsufficient to pull armature 18 and the flange 21 upward against thetention of the thermostat.

One of the terminals 22 of a string of series connected lamps isconnected to the terminal screw 16 while the other is connected to-theterminal screw 17 preferably by looping the ends around the shanks ofthe screws before they are tightened into place, although other securingmeans may be used. The terminal wires 22 pass out through a smallaperture in the end of a bell shaped cover 23 which has an inner leftend diameter adapting it to fit tightly over the recessed end portion ofms ing 4 where its left end abuts against an annular shoulder 24 at theinner end of the recessed section, thereby making a smooth continuousouter surface at the point of juncture. The cover is held in thisposition by friction, thus making the terminals and series cut-outaccessible when installing or repairing.

Fig. 3 shows a cut-out similar to that above described with theexception that the thermostat 14 (Fig. 2) is replaced by a spring 25comprising two metallic sections held in spaced relation by a piece-ofinsulating material 26 to which the adjacent ends are riveted. Theelectrical circuit through the spring is completed through a coil orsolenoid 27 which surrounds the armature 18. The field strength ofsolenoid 27, when the normal amount of current is flowing through it, isnot suflicient to create a magnetic attraction between flange 21 andmagnet 20 which is strong enough to bend spring 25 upward, but a smallincrease of current, such as would result from the burning out of a lampwill increase this magnetic attraction sufliciently to overcome thetension of the spring 25 and pull it upwardsufliciently for the flange21 to make contact with the fixed magnet 20, which is then strong enoughmagnetically to attract the flange with a force greater than the tensionof the spring pulling downward.

The' operation of the series cut-out is as follows: When one of thelamps 28 (Fig. 1)

burns out, the instantaneous potential rise {resulting therefrom breaksdown the resistance barrier in the cut-out 29 which is disposed betweenthe terminals of the lamp burning out and which completes a circuitby-passing the burnt out lamp filament. Due

to the fact that the resistance of the circuit is now reduced by anamount equal to the resistance of the burnt out filament, an increase 5of current proportional to this'decrease flows through thermostat 14causing it to bend upward which brings flange 21 into contact with thering magnet 20 which is stron enough magneticallyto hold the flange anthermostat in this position. When thermostat 14 moves'upward it breakscontact with spring 15 thus putting the resistance wire 7 in series withthe lamp circuit, which -pro ortionally reduces the amount of currentowing through the remaining lamps. When a new lamp is substituted inplace of the one burned out, armature 18 is ressed downward to againcomplete the circuit through the thermostat 14 and spring 15. a

The cut-out shown in Fig. 3 works in the same manner with the exceptionthat the upward bending of spring 25 is due to the increased magneticattraction created by the increased flow of current through the solenoid27.

While I have shown, illustrated and described certain embodiments of myinvention, it will be understood that it is not limited to the exactdetails shown, but is capable of modification and variation within thespirit of the invention and within the scope of the appended claims.

What I claim' as new and desire to secure byLetters Patent of the UnitedStates, is:

1. In combination, a string of incandescent lamps connected in series, acut-out connected across the filament terminals of each lamp, aresistance, and means actuated by the increase in current caused by theburning out of any one of said lamps and the .operation of its cut-out,to efiect the insertion of said resistance into the series circuit.

2. In combination, a string of incandescent lamps connected in series, acut-out connected across the filament terminals of each-lamp, aresistance, and a thermostatic contact member actuated by the increasein current caused by the burning out of any one of said lam s and theoperation of its cut-out, to effect t einsertion of said resistance intothe series circuit. I

3. The combination with a string of incandescent lamps connected inseries, and a cut one of said terminals contacts carried by the body andnormally iorming a shunt circuit around said resistance, and currentactuated means for separating said contacts.

5. A wiring device comprising a body having a pair of plug terminals, aresistance carried by' the body, and means carried by the body foreifecting automatically the insertion of said resistance in series withone of said terminals when the current flowin through the terminalsexceeds a predetermined high value.

In witness whereof, I have hereto set my hand this 20th day of Ma 1930.

HENR BUTLER.

out connected across the filament terminals of I each lamp, of acombined \plug terminal and resistanceconnected to said string of lamps,said resistance being normally disconnected from-the series circuit, andmeans actuated by the increase in current caused by. the burning out ofany one of said lamps and the operation of its cut-out for efiecting theinsertion of said resistance into the series circuit.

i -A wiring device comprising a body having a pair of plug terminals, aresistanc'ecara5 ried by the body and connected in serieswith

